Tractable nonlocal correlation density functionals for flat surfaces and slabs

A systematic approach for the construction of a density functional for van der Waals interactions that also accounts for saturation effects is describ ed, i.e., one that is applicable at short distances. A very efficient metho d to calculate the resulting expressions in the case of flat surfaces, a me thod leading to an order reduction in computational complexity, is presente d. Results for the interaction of two parallel jellium slabs are shown to a gree with those of a recent, more elaborate calculation [J.F. Dobson and J. Wang, Phys. Rev. Lett. 82, 2123, (1999)]. The method is easy to use; its i nput consists of the electron density of the system, and we show that it ca n be successfully approximated by the electron densities of the interacting fragments. Results for the surface correlation energy of jellium compare v ery well with those of other studies. The correlation-interaction energy be tween two parallel jellia is calculated for all separations d, and substant ial saturation effects are predicted.